We will start by creating one link, which we will repeat with a stroke around a path. Create a new document, I'm using 70x70px. Then either paint a black circle in the middle or use the elliptical marquee tool to create a circle on a new layer like so:
Then choose the eraser tool and make a donut out of the circle, like so:
Now press Ctrl+A to select all and then goto Edit > Define Brush Preset. Give it a name and click Ok. You can now close this document if you so wish.
Open a new document of 400x300px, and choose the brush tool. Click the brushes drop-down menu and the one we just created should be at the bottom of the list - select it. Now choose the Pen tool and draw a Path that resembles the chain you want to create. We use the Pen to set the general shape, and then the Convert Point tool to create the curves:
Now choose the Brush tool again, making sure our custom brush is selected, then goto the Brush options and make sure Spacing is set to 100%. We may also want to reduce the diameter of the brush to around 20px:
Next, create a new layer in the Layers Window, and go to the Paths Window (Window > Paths). Right-click the Path we just created and choose Stroke Path. We want to Stroke it with the Brush, so click Ok:
You may need to play around with the length of your Path or the diameter of your Brush if all of your links aren't fitting on nicely, particularly if you're creating a closed link.
We should have something like this:
Go back to your Layer Window and then Ctrl-Click the layer to make a selection. Goto Filter > Render > Clouds, with Black and White selected in the color palette. Then goto Filter > Sketch > Bas Relief. Play with the settings until you find a good look:
Next, create a new layer and then choose the Brush tool. Change the Brush to a round 3-4px, hard-edged brush. Then go back to the Paths Window, right-click the Path and choose to Stroke it, again. Make sure there are no Dynamics applied to the brush in the brush settings. This will create a stroke down the middle of the Path on a new layer:
We can now delete this Path by right-clicking it and choosing delete, then go back to the Layer Window and apply the same filters to the new stroke - Filter > Render > Clouds (after, making the selection), then Filter > Sketch > Bas Relief.
The final step is to erase the middle parts of the last stroke to create the links. Choose the eraser with a brush size of the middle (empty) part of the chain link. Then we simply erase the middle part of the stroke:
We end up with a chain looking something like this:
